A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The pattern to be transferred to the substrate may include very small structures. Any mechanical disturbance such as a vibration or the like may result in an erroneous transfer rendering the substrate unusable. To minimize a transfer of vibrations and other mechanical disturbances from any external structure, such as a support structure or floor, to the lithographic apparatus, in particular a substrate table, a pattern support structure and/or a projection system thereof, it is known to employ one or more vibration isolation support devices to support the lithographic apparatus.
Such a vibration isolation support device supports the lithographic apparatus, i.e., at least partly compensates a gravitational force exerted thereon. Thus, the vibration isolation support device provides a mechanical connection between the lithographic apparatus and an external structure, e.g., a floor. Depending on a frequency of the vibration, a vibration present in the external structure may be (partly) damped by the isolation support device and/or (partly) transferred from the external structure to the lithographic apparatus. The amount of damping and/or transfer of a vibration of a certain frequency depends on a stiffness of the vibration isolation support device. A relatively small stiffness results in relatively high vibration isolation. Therefore, it is desirable to have a vibration isolation support device having a small stiffness.
A well-known vibration isolation support device is an airmount. Such an airmount includes a gas chamber containing an amount of pressurized air and a moveable member partly positioned in the gas chamber. The pressure of the air exerts a support force on the moveable member. An object such as a lithographic apparatus, or a part thereof, is supported on the moveable member. The known airmount has such a (positive) stiffness that it is suitable to isolate vibrations above a predetermined frequency such that a pattern having a predetermined minimum feature size may be correctly transferred. However, there is a need to transfer smaller structures having smaller feature sizes and therefore, there is a need for a vibration isolation support device having a smaller stiffness.